Ratings of water filters that screen out particulate are usually stated in micron size. The micron is a standard measure of size that is used by filter makers. The diameter of a human hair is about 90 microns. Sediment filters are used to catch particles 1/300 of that.

Most sediment filters are given ratings by their manufacturers that describe their effectiveness at removing particles down to a specified size. The most common of these are”nominal” and “absolute.”

Nominal, according to the Water Quality Association (WQA), means that the filter will filter out at least 85% of the particles of the size it is rated for. In other words, a filter that is rated as a 1 micron nominal can be expected to pick out 85% of the particles that are 1 micron or larger from the water that passes through it.

Absolute, theoretically, means that the filter will reject virtually all of the particles of the given size. The usual expectation is a 3-log rejection–or 99.9%. Absolute ratings are usually used for the tightest filters and for purposes where efficiency really matters. For example, if a filter maker promises removal of E. coli, more or less 85% efficiency isn’t good enough. If you’re going to trust your life to the filter, you expect an absolute 3-log or 4-log rating at the very least.

The problem with the absolute vs. nominal system is that there is really no universal standard that assures uniformity. Some makers of filters for non-critical applications, for example, might consider 70% rejection suitable for a nominal filter. Definitions vary from one manufacturer to another, and there is really no way for the end user to verify the claim.

Beta Ratio is less frequently used. It establishes a ratio between the particles that are retained and the particles that pass through the filter. The formula is Particles In divided by Particles Out. Thus, the higher the Beta Ratio rating the better. A beta ratio of 1000 would mean that the filter allows only one particle per 1000 to pass through. A beta ratio of 100 means one particle per 100. The 1000-rated unit, therefore, is 10 times as effective. Note, however, that the beta ratio is no guarantee that the filter will perform as well in areas other than what it was tested for. The filter that works best in one application may not be effective in another.

Here are suggested maximum flow rates in gallons-per-minute for the four most common cartridge sizes in the popular Flow Max series of pleated sediment cartridges.